Drive apparatus for PWM control of two inductive loads with reduced generation of electrical noise

ABSTRACT

A drive apparatus for driving two motors by PWM control of respective switching elements connected to the motors, whereby respective control signals are applied to the switching elements such that each commencement of a transition of one of the switching elements from the non-conducting to the conducting state coincides with the termination of a transition of the other switching element from the conducting to the non-conducting state, thereby reducing generated electrical noise, and for supplying equivalent values of drive voltage to two motors of different power ratings which rotate respective cooling fans, to obtain equalized levels of air flow rate.

BACKGROUND OF THE INVENTION

1. Field of Application

The present invention relates to an inductive load drive apparatus forapplying PWM (pulse width modulation) control to mutually separatelydrive a pair of inductive loads that are connected to a DC power source.

2. Description of Prior Art

In the prior art, various methods have been proposed for overcoming theproblems that arise when a plurality of loads are driven in parallel,with the driving being based on PWM control signals. For example withJapanese patent 2002-43910, it is ensured that each falling edge of aPWM control signal that controls driving of one of the loads coincideswith a rising edge of the PWM control signal that controls driving ofthe other load, for thereby ensuring that an increase in load current ofone of the loads will be cancelled by a decrease in load current of theother load, and so reduce variations in the overall load current. Thatis to say, if both of the loads were to be supplied with load currentconcurrently at any time, then a large increase in the overall loadcurrent would occur. However with that invention, by establishing ashift between the conduction timings of the two loads, it is ensuredthat the degree of variation of the overall load current is minimized.

Such a technique has been successfully applied in the case of resistiveloads, such as the headlamps of a vehicle. However it would be desirableto be able to apply a similar method to driving inductive loads, such aselectric motors. When a switching element such as a FET (field effecttransistor) is used to control current flow through an inductive loadfrom a DC power source such as a battery, with a rectangular-waveformPWM control signal being applied to the switching element, then eachtime the switching element is set in the off state, a regenerative(i.e., reverse-polarity) current flow occurs from the inductive loadback into the power source. As a result, the waveform of power sourcecurrent flow is approximately sinusoidal.

FIG. 7 is a comparative example for illustrating how it might beattempted to adapt the technique of the above-mentioned Japanese patentto the case of driving a pair of inductive loads, which will be assumedto be respective motors. In FIG. 7, a series combination of a motor 2Aand MOS FET 2A and a series combination of a motor 2B and MOS FET 2B areconnected in parallel between the potential of a DC power sourceconsisting of a battery 1 and ground potential. More specifically, thedrains of the MOS FETs 2A, 2B are connected through respective diodes4A, 4B to one side of a π-configuration filter (referred to in thefollowing simply as a π filter) 5, with the other side of the π filter 5connected to the potential of the battery 1, and with the diodes beingconnected in a direction such as to be reverse-biased when thecorresponding MOS FET is set in the on state. (It should be noted thatneither the diodes 31A, 31B nor the π filter 5 are described in theaforementioned Japanese patent 2002-43910, which describes the drivingof resistive loads only).

When a MOS FET 3A or 3B is switched from the on to the off state, aresultant regenerative current flows from the corresponding one of themotors 2A, 2B through the corresponding one of the diodes 4A, 4B intothe battery 1. The π filter 5, which is formed of a coil 8 andcapacitors 6, 7 as shown, serves to absorb these flows of regenerativecurrent and thereby smooth out fluctuations in the potential of thebattery 1 which would otherwise result from such flows of regenerativecurrent.

A pair of PWM control signals A and B having identical duty ratio,produced from a control IC 9, are applied through respective drivecircuits 10A, 10B as respective PWM to the gates of FETs 3A, 3Brespectively. When a FET 3A or 3B is set in the on state, current flowsfrom the battery 1 through the corresponding one of the motors 2A, 2Band that FET to ground potential. When that FET 3A or 3B is then set inthe off state, a delayed current flows through the corresponding one ofthe diodes 4A, 4B to the π filter 5, to return to the battery 1 as aregenerative current. This regenerative current flow is smoothed by thecapacitor 6 of the filter 5. The capacitor 7, on the opposite side ofthe π filter 5 from the diodes 4A, 4B, serves to smooth fluctuations inthe potential of the battery 1.

However with such a configuration, the level of the regenerative currentis substantial, so that it is necessary to use a large value ofcapacitance for the capacitor 6. In order to minimize the necessarycapacitance of the capacitor 6, it is desirable to reduce the peak levelof the regenerative current as far as possible.

FIGS. 8A, 8B illustrate the effect of changing the phase differencebetween the PWM control signals A and B upon the waveform of a ripplecomponent which appears in the supply current of the battery 1 with thecircuit of FIG. 7, for the case in which the PWM control signals are ofapproximately rectangular waveform. The ripple component is measured atthe positive terminal of the battery 1.

FIG. 8A shows the power supply voltage waveforms for MOS FETs 3A, 3B asmeasured at points A, B in FIG. 7, when the PWM control signals are ofidentical phase. In that case the motors 2A, 2B are supplied with powerconcurrently, and hence a large amplitude of ripple appears in the powersource current from the battery 1. However if a suitable phasedifference is established between the control signals A and B, such asto ensure that the motors 2A, 2B are not supplied with currentsimultaneously from the battery 1, as shown in FIG. 8B, then the amountof ripple can be substantially reduced.

A problem arises with such a method whereby the motors 2A, 2B are drivenalternately as shown in FIG. 8B, in that the frequency of the ripplecomponent in the supply current of the battery 1 is doubled, bycomparison with the case in which the motors are driven concurrently. Asa result, electrical noise is generated at the frequency of that ripplecomponent. The overall noise level of the vehicle electrical system isthereby increased.

It might be envisaged that this problem could be overcome by forming thecontrol signals with a trapezoidal waveform and arranging that the startof each rising edge of one of the control signals A and B is made tocoincide with the start of a falling edge of the other one of thesesignals, as illustrated in the timing diagram of FIG. 9A. It wouldappear that such a method could substantially reduce the level of theripple component. However in practice, due to the intervals in whichboth of the FETs 3A, 3B are conducting simultaneously, that is to saywhen a turn-on interval (i.e., transition interval from thenon-conducting to the conducting state) of one FET overlaps a turn-offinterval (i.e., transition interval from the conducting to thenon-conducting state) of the other FET as illustrated in FIG. 9A,distortion of the waveform of the ripple component of the supply currentof the battery 1 is produced. This waveform distortion increases thelevel of electrical noise.

Another problem which arises in the prior art with respect to driving aplurality of motors constituting respective inductive loads is asfollows. In certain applications, such as driving the cooling fans whichdirect air flows into the engine radiator and into the condenser of theair conditioner system of a motor vehicle, it is desirable thatidentical rates of air flow are produced by each of the cooling fans, inorder to maximize the efficiency of the cooling operation. Thus, forexample in the case of using two motors in parallel in such anapplication, it would be preferable to be able to utilize two motorswhich have identical values of power rating, for example which are bothrated at 100 W, or are both rated at 200 W, to ensure that identicallevels of power are produced by the motors when they are driven byrespective PWM voltages of identical duty ratio. However it might benecessary that the maximum amount of output power that will be produced(in total) by the motors is for example to be 260 W. In such a case,since it is very possible motors having a power rating of 130 W may notbe available, it might in practice be necessary to use a pair of motorswhich have respectively different values of power ratings, for example acombination of a 100 W motor and a 160 W motor. However with prior artmethods of PWM drive control of such a combination of motors by using acommon value of PWM duty ratio for both motors (and so, identical levelsof average drive voltage) this would result in unbalanced amounts ofoutput power being produced by the motors. Such cases of having toutilize an unbalanced combination of motors are very frequent in theprior art, and lead to inefficiency in such applications as driving thecooling fans of a vehicle as described above.

SUMMARY OF THE INVENTION

It is an objective of the present invention to overcome the aboveproblems of the prior art by providing an inductive load drive apparatusfor application to driving two inductive loads mutually separately,whereby only small values of capacitance are required for capacitorsthat are used in constituting a filter such as a π-configuration filterwhich is incorporated for absorbing regenerative current flow, andwhereby generation of electrical noise due to fluctuations in the supplycurrent of a DC power source such as a vehicle battery which powers theapparatus can be held at a low level.

To achieve the above objective, according to a first aspect, theinvention provides an inductive load drive apparatus for mutuallyindependently driving two inductive loads that are connected to a DCpower source, by PWM operation, with the apparatus having a controlcircuit that includes two switching elements which are respectivelyconnected in series between the two inductive loads and the DC powersource, each of the switching elements controllable for being switchedto an on state and an off state for thereby enabling and inhibiting asupply of a drive current from the power source through thecorresponding one of the inductive loads, a π-configuration filterformed of an inductor and two capacitors, coupled to the pair ofswitching elements for providing a flow path for a regenerative currentwhich flows to the power source from an inductive load when a switchingelement corresponding to the inductive loads is set in the off state.The control circuit is characterized in comprising phase control meansfor controlling generation of the two PWM control signals such that eachtiming of termination of each first-direction edge of a specific one oftwo PWM control signals that respectively control the switching elements(i.e., an edge direction whereby the corresponding switching element ischanged to the off state) coincides with a timing of the start of asecond-direction edge of the other one of these PWM control signals(i.e., and edge direction whereby the corresponding switching element ischanged to the on state).

By establishing such a phase relationship between the two PWM controlsignals, it is ensured that the two switching elements can never besimultaneously set in the on state. The level of ripple in the supplycurrent from the power source, and amount of distortion (i.e., inrelation to a sinusoidal waveform) of residual ripple in that supplycurrent, can thereby be made very small. Hence, generation of electricalnoise is suppressed, and it is only necessary to use relatively smallvalues of capacitance for the capacitors which constitute the n filter.

If the PWM control signals have a high frequency, then these signalsthemselves may become a source of electrical noise. For that reason,according to another aspect, the control circuit includes waveformshaping circuit means for shaping each of the two PWM control signals tohave a trapezoidal waveform. In that way, each of the transitionsbetween the high and low levels of each of the PWM drive voltagesproduced by the switching elements is made more gradual than is the casefor a rectangular waveform, and this is effective in reducing thegeneration of electrical noise.

Moreover by forming the waveforms of the PWM control signals with atrapezoidal shape, whereby at each transition, a PWM control signalgradually rises to a level at which the corresponding switching elementis set completely in the on (i.e., conducting) state, or gradually fallsto a level whereby that corresponding switching element is set in theoff (i.e., non-conducting) state, the duration of each of the intervalsin which a switching element remains in the on state is made longer withrespect to the duration of each interval in which the switching elementis in the off state. By applying such a feature to an apparatusconfigured according to the first aspect of the invention describedabove, it becomes possible to ensure that the respective intervals inwhich the two switching elements are in the on state will nevercoincide, irrespective of the value of duty ratio that is establishedfor the PWM control signals. Hence, further effectiveness in reducingthe generation of electrical noise can be achieved.

It is a further objective of the invention to overcome the problem ofthe prior art described hereinabove whereby unbalanced levels of outputpower are produced by respective inductive loads constituted by aplurality of motors, such as motors which operate cooling fans of amotor vehicle, which are each controlled by a prior art type of PWMdrive apparatus.

To achieve the latter objective, the invention provides an inductiveload drive apparatus for driving of at least two inductive loadsconstituted by respective motors having mutually different power outputratings, by PWM control of respective switching elements connected tothe motors, wherein the apparatus comprises a voltage compensationcircuit for modifying respective PWM control signals applied to theswitching elements such as to apply respectively different values ofdrive voltage to the motors, with the respectively different values ofdrive voltage being predetermined in accordance with the respectivepower output ratings of the motors.

In the case of an application in which the motors drive cooling fans,the respectively different values of drive voltage are predetermined inaccordance with the respective power output ratings of the motors suchas to ensure that identical values of air flow are produced by thecooling fans.

In general, such an inductive load drive apparatus receives an inputdrive command signal from an external source, having a parameter value(e.g., PWM duty ratio) that expresses a required level of total outputpower from the combination of motors. With the present invention, foreach of two or more motors which may have respectively differentratings, a voltage compensation circuit derives a compensation voltageby obtaining the average value of drive voltage that is currently beingapplied to the motor and multiplying that value by a presettableamplification factor. The drive command signal is expressed as a drivecommand voltage (e.g., by conversion from a PWM signal, if necessary)and the compensation voltage is applied to modify that drive commandvoltage. The resultant compensated drive command voltage is applied todetermine the duty ratio for operating the switching element of thatmotor, and thereby determine the average value of drive voltage appliedto the motor. The amount of compensation thereby applied for each motoris determined such that respectively equivalent values of average drivevoltage are applied to the motors, determined such that substantiallyidentical levels of output power (e.g., measured as respective values ofair flow rate from fans which are driven by the motors) are produced bythe motors, irrespective of changes in the input drive command signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a general system block diagram of a first embodiment of aninductive load drive apparatus;

FIG. 2 is a circuit diagram for describing a phase processing section ina control circuit in the first embodiment;

FIG. 3 is a timing diagram for use in describing a phase relationship ofPWM drive voltages with the first embodiment;

FIGS. 4A to 4C are examples of measured waveforms of PWM control signalsand of supply current ripple;

FIG. 5 is a bar chart for illustrating a relationship between the dutyratio of a PWM drive voltage applied to a motor and an effective levelof current which flows in a capacitor of a π filter, for the case of theprior art and for the case of the first embodiment respectively;

FIG. 6 is a timing diagram of an example of PWM drive voltages for thecase of an alternative configuration in which PWM control signals areapplied in push-pull form;

FIG. 7 is a general system block diagram of a comparative example, whichis a possible modified configuration of a prior art type of PWM loaddrive apparatus;

FIGS. 8A, 8B are waveform examples corresponding to those of FIGS. 4A to4C, for illustrating the relationship between supply current ripple anda phase difference between respective PWM drive voltages of two motors;

FIG. 9A is a timing diagram for illustrating a phase relationshipbetween PWM drive voltages with a prior art type of PWM load driveapparatus, and FIG. 9B is a timing diagram showing a condition in whichtwo PWM drive voltages for respective motors are of identical phase;

FIG. 10 is a general system block diagram of a second embodiment of aninductive load drive apparatus, which drives motors having respectivelydifferent ratings;

FIG. 11 is a circuit block diagram showing details of drive voltagecompensation sections of a control circuit in the second embodiment;

FIG. 12 is a graph illustrating relationships between values of drivevoltage applied to respective motors having different ratings andresultant values of air flow rate produced by fan that are driven by themotors;

FIG. 13 is a timing diagram of an example of PWM control signals havinga rectangular waveform.

DESCRIPTION OF PREFERRED EMBODIMENTS

A first embodiment will be described in the following, which drivesinductive loads consisting of a pair of motors. It will be assumed thatthe motors drive respective cooling fans (not shown in the drawings) forcooling the engine radiator and the condenser of the air conditionersystem in a motor vehicle. FIG. 1 is a general system block diagramshowing the overall configuration of this embodiment. This is based on acontrol circuit 100 which is essentially similar to the control circuitin the comparative example of FIG. 7 described above. However thecontrol circuit 100 differs by the incorporation of a phase processingsection 35, as described hereinafter. Specifically, the control circuit100 is formed of an input signal processing section 34, the phaseprocessing section 35, drive circuits 36A, 36B, N-channel MOS FETs 23A,23B, diodes 24A, 24B, and a π filter 25, as its main elements.

A battery 21 constitutes a DC power source, and motors 22A, 22Bconstitute respective inductive loads, which are driven by PWM operationby the control circuit 100. The motor 22A and MOS FET 23A are connectedin series between the (positive) supply potential of the battery 21 andground potential, while the motor 22B and MOS FET 23B are similarlyconnected in series between the supply potential of the battery 21 andground potential. The drains of the MOS FETs 23A, 23B are connectedthrough respective diodes 24A, 24B to one side of the π filter 25, withthe other side of the filter 25 connected to the positive potential ofthe battery 21, and with the diodes being connected in a direction suchas to be reverse-biased when the corresponding one of the MOS FETs 23A,23B is set in the on (i.e., conducting) state. Capacitors 31A. 32 fornoise suppression are respectively connected in parallel with the diodes24A, 24B. The source terminals of the MOS FETs 23A, 23B are connected toground potential.

The π filter 25 is formed of a coil 28 and capacitors 26, 27 as shown.

An engine ECU, for controlling the engine of the vehicle, is based on amicrocomputer. A drive control signal, which specifies the duty ratio ofPWM control signals as described in the following, is supplied from theengine ECU to the input signal processing section 34 of the controlcircuit 100. The drive control signal will be assumed to be alow-frequency PWM signal, having a frequency of approximately 100 Hz,whose duty ratio is set in accordance with a required level of combinedoutput power from the motors 22A, 22B. The input signal processingsection 34 converts that drive control signal to a corresponding drivecommand voltage, which directly determines the duty ratio of the PWMdrive voltages applied to the motors 22A, 22B.

As shown in FIG. 2 the phase processing section 35 is formed of acarrier signal output section 35 a which produces a sawtooth-waveformcarrier signal, having a frequency of approximately 19 kHz for example,a phase shift section 35 b and a pair of comparators 300A, 300B. In thephase shift section 35 b, the path of the carrier signal from thecarrier signal output section 35 a is split into two paths, with aspecific amount of phase shift being applied by a phase shift circuit 35c in one path, and with the resultant phase-shifted output being appliedto the inverting input terminal of the comparator 300A and the nonphase-shifted carrier signal being applied to the inverting inputterminal of the comparator 300B. The drive command voltage is applied tothe respective other input terminals of the comparators 300A, 300B, tobe thereby compared with the direct carrier signal and the phase-shiftedcarrier signal. Respective PWM control signals A and B, each having aduty ratio determined by the drive command signal from the input signalprocessing section 34, are thereby outputted from the comparators 300A,300.

These PWM control signals A and B are respectively supplied through thedrive circuits 36A, 36B to resistors 37A, 37B respectively, andresultant waveform-shaped PWM control signal A and B are supplied fromthe resistors 37 a, 37B to the gates of the MOS FETs 23A, 23Brespectively.

The operation of this embodiment is as follows. The paths of thecurrents that flow through the motors 22A, 22B when the correspondingMOS FETs 23A, 23B are switched on and off are identical to thosedescribed for the comparative example of FIG. 7 hereinabove. That is tosay, when a MOS FET 23A or 23B is switched to the on state, currentflows from the battery 21 through the corresponding one of the motors22A, 22B and that MOS FET 23A or 23B, to ground potential, with themotor being driven thereby. When a MOS FET 23A or 23B is switched to theoff state, regenerative current flows from the corresponding one of themotors 22A, 22B through the corresponding one of the diodes 24A, 24B andthen through the filter 25, back to the battery 21. The flow ofregenerative current is smoothed by the capacitor 26 of the filter 25.

The input PWM signals produced from the phase processing section 35 aretransferred through the series-connected resistors 37A, 37Brespectively, for thereby converting these to respective waveform-shapedPWM control signals A and B, having a trapezoidal waveform, which aresupplied to the gates of the MOS FETs 23A, 23B respectively, with thewaveform-shaped PWM control signals A and B having a specific amount ofphase difference between them as determined by the phase control section35 b. This is illustrated in the timing diagram of FIG. 3, in whichrespective drive voltages of the MOS FETs 23A, 23B resulting fromapplying the waveform-shaped PWM control signals are designated as PWMdrive voltage A, PWM drive voltage B. As shown, the aforementioned phasedifference is established such that the point of termination of eachfalling edge of a specific one of these PWM signals (i.e., sinceP-channel MOS FETs are used in this embodiment, the end of a turn-offinterval of the corresponding MOS FET) coincides with the point ofcommencement of a rising edge of the other one of the signals, so that aturn-on interval of one MOS FET begins immediately after a turn-offinterval of the other MOS FET has terminated.

Thus, by establishing such a phase relationship for the PWM controlsignals, each time that the MOS FET 23B begins to be turned on, the MOSFET 23A has already completed the process of being turned off. Thisensures that the amount of ripple in the supply current of the battery21 is reduced to a low amplitude, thereby reducing the level ofelectrical noise that is generated by the operation of the inductiveload drive apparatus. In addition, a condition is avoided whereby bothof the MOS FETs 23A, 23B change from a state in which both of these areoff to a state in which one FET is changing from off to on while theother FET is changing from on to off, as described above referring toFIG. 9A. Thus, waveform distortion of the residual amount of ripple inthe supply current of the battery 21 is suppressed.

In addition to the above conditions being satisfied by the phaserelationship between the switching of the MOS FETs 23A, 23B, preferablya further condition should be satisfied, whereby when one of the MOSFETs 23A, 23B enters the on state, the other FET remains in the offstate for a substantially long interval (i.e., an interval which shouldbe as long as possible with respect to the duration of the interval forwhich the first-mentioned FET remains in the on state). If thatcondition is satisfied, then the level of regenerative current whichflows in the capacitor 26 of the filter 25 will be lowered. Thatcondition is satisfied by forming the waveform-shaped control signalswith a trapezoidal waveform as described above.

That is to say, during an interval in which one of the MOS FETs 23A, 23Bis in the on state and the other is in the off state, regenerativecurrent flows into the capacitor 26 from the one of the motors 22A, 22Bthat is controlled by the FET which is in the off state. However at thesame time, current is being supplied from the battery 21 to the one ofthe motors 22A, 22B that is controlled by the FET which is in the onstate. That flow of current acts to compensate for the regenerativecurrent flow of current into the capacitor 26, i.e., acts in a directiontending to discharge the capacitor 26. Thus, since these two flows ofcurrent into the capacitor 26 serve to mutually cancel, the effectivelevel of regenerative current that flows in the capacitor 26 is reduced.

FIGS. 4A, 4B, 4C illustrate the results of oscilloscope measurement ofthe ripple waveform of the supply current of the battery 21 (measured atthe positive terminal of the battery 21) in relation to changes in thephase relationship between the aforementioned PWM control signals A andB, with these signals being designated in FIGS. 4A to 4C as PWM (A) andPWM (B) respectively. As shown in FIG. 4A, if the transitions betweenthe on to off and off to on states of the MOS FETs 23A, 23B are keptcompletely separate, with no overlapping, then a large amount ofdistortion occurs in the ripple waveform. FIG. 4B illustrates the casein which there is a partial overlap between these transitions, i.e.,during each transition of MOS FET 23A from the off to the on state, atransition of MOS FET 23B from the on to the off state occurs. As shown,a significant amount of distortion of the ripple waveform remains.However as shown in FIG. 4C, if the aforementioned phase conditions ofthe present invention are established for the PWM control signals A andB whereby each timing of the start of turning on one of the FETs (inthis example, MOS FET 23A) coincides with the timing of termination ofturning off the other one of the FETs (in this example, MOS FET 23B),then the distortion of the ripple waveform is effectively suppressed.

FIG. 5 is a bar chart which illustrates the effective level of currentwhich flows in the capacitor 26 of the π filter 25, for the case of aninductive load drive apparatus using PWM control signals to which thephase control of the above embodiment is not applied, and for the caseof using PWM control signals having a phase relationship as describedfor the above embodiment. Here, the term “phase control is not applied”signifies that the two PWM control signals are of identical phase, asillustrated in the waveform diagram of FIG. 9B. The measurementconditions are that the power source voltage is 15.1 V, there are two200 W loads, the PWM frequency is 19 kHz, and the PWM duty ratio isvaried from 30% to 70% in steps of 10%. As can be understood from thisdiagram, by using the control method of this embodiment, the level ofcurrent which flows in the capacitor 26 is made substantially lower thanfor the prior art, for all values of duty ratio.

If the duty ratio of the two PWM control signals is close to 50%, thenwith the above embodiment, occurrence of coincidence between the timingof the start of a falling edge of a first one of the PWM control signalsand the timing of the start of a rising edge of the second PWM controlsignal will occur in alternation with occurrence of coincidence betweenthe timing of the start of a falling edge of the second PWM controlsignal and the timing of the start of a rising edge of the first PWMcontrol signal.

That is to say, if the duty ratio is less than 50% then the controlsignals are applied such that for (an arbitrarily determined) one of thetwo switching elements, each transition of that switching element fromthe on (i.e., conducting) state to the off (i.e., non-conducting) statecoincides with the commencement of a transition of the other one of theswitching elements from the off to the on state. If the duty ratio is50%, then that condition applies to both of the switching elements.

The term “first-direction edge” of a control signal as used in theappended claims has the special significance of “a transition of thatcontrol signal in a direction whereby the corresponding controlledswitching element is changed from the on to the off state”. Similarly,the term “second-direction edge” has the special significance of “atransition of that control signal in a direction whereby thecorresponding controlled switching element is changed from the off tothe on state”.

As can be understood from the above, with this embodiment, the two PWMcontrol signals A, B that are applied to control the P-channel MOS FETs23A, 23B respectively, with a duty ratio determined by the drive commandsignal supplied from the engine ECU, are configured such that each startof a falling edge of one of the PWM control signals coincides with thetiming of the termination of a rising edge of the other one of the PWMcontrol signals. As result, in addition to reducing the level of ripplein the supply current, distortion of the waveform of that ripple is alsosubstantially reduced. Hence, the capacitor 26 of the π filter 25 needonly have a small value of capacitance, while enabling electrical noisethat is generated by operation of the inductive load drive apparatus tobe substantially reduced by comparison with the prior art.

Furthermore, by incorporating the resistors 37A, 37B to perform awaveform shaping function, it is ensured that each of thewaveform-shaped PWM control signals A and B applied to the gates of theMOS FETs have a trapezoidal waveform, so that each of these signalsrises and fall gradually. This further assists in reducing the level ofelectrical noise which is generated by operation of the apparatus.Utilizing such a waveform results in an increased duration of each ofthe turn-on and turn-off intervals of the MOS FETs 23A, 23B. However dueto the action of the phase processing section 35, it is ensured thatoverlapping of these turn-on and turn-off intervals of respective FETsis avoided. Hence, lowering of the overall electrical noise iseffectively achieved.

It should be noted that various modifications to the above embodimentcould be envisaged. For example, with the above embodiment, each of thePWM control signals A, B is applied by single-ended output from a drivecircuit, through a resistor to the gate of the corresponding switchingelement. However it would be equally possible to use a push-pull type ofdrive circuit, in which one output of a drive circuit (i.e., the push-upoutput) is connected through a first resistor to the gate of thecorresponding MOS FET and a second output (i.e., the pull-down output)is connected through a second resistor to that gate. In that case, asillustrated in FIG. 6, it may be preferable to make the respectivedurations of each turn-on interval and turn-off interval different fromone another, by setting respective appropriate values for theabove-mentioned first and second resistors.

Furthermore the above embodiment has been described for the case ofusing trapezoidal PWM control signals, however, depending upon the levelof electrical noise that is permissible, it may be possible to use arectangular waveform.

Moreover the invention is not limited to the use of MOS FETs asswitching elements, and is equally applicable to various other types ofcontrolled switching elements such as bipolar power transistors, IGBT(insulated gate bipolar transistors), etc.

Furthermore it would be equally possible to connect these as high-endswitching elements, i.e., each connected between the high potential ofthe power source (e.g., B+ potential, with the above embodiment) and thecorresponding motor.

Moreover the invention is not limited to the case of driving inductiveloads that are motors which drive cooling fans of a vehicle, but isapplicable in general to cases in which two inductive loads must bedriven mutually independently by PWM operation.

A second embodiment will be described, referring first to the generalsystem block diagram of FIG. 10. As for the first embodiment, the secondembodiment is an inductive load drive apparatus mounted in a motorvehicle, which drives a pair of inductive loads consisting of respectivemotors 122A, 122B, which have respectively different power ratings. Withthis embodiment the motors 122A and 122B are coupled to drive respectivecooling fans 251, 252, which direct a flow of air into the engineradiator (not shown in the drawing) and the condenser (not shown in thedrawing) of the air conditioner system of the vehicle. Although theprinciples of this embodiment are applicable to control of the outputpower of electric motors which drive various types of load, it should beunderstood that in the following description of the embodiment, the term“output power produced by a motor” signifies the output power measuredas a rate of airflow that is generated by a fan which is driven by themotor.

The embodiment basically consists of a control circuit 110 which ispowered by the battery 21 of the vehicle and receives a drive commandsignal from the engine ECU 32 of the vehicle. The drive command signalis assumed to be a low-frequency PWM signal whose duty ratio isindicative of a required rate of flow of cooling air that is to besupplied by the cooling fans 251, 252 in combination.

The control circuit 110 is formed of an input signal processing section34, a phase processing section 135, drive circuits 36A, 36B, drivevoltage compensation sections 113A, 113B, MOS FETs 23A, 23B, diodes 24A,24B, and a π filter 25.

The motors 122A, 122B are driven by PWM operation by the control circuit110, with the motor 122A and MOS FET 23A being connected in seriesbetween the (positive) potential of the battery 21 and ground potential,and the motor 122B and MOS FET 23B similarly connected in series betweenthe positive potential of the battery 21 and ground potential. Thedrains of the MOS FETs 23A, 23B are connected through respective diodes24A, 24B to one side of the π filter 25, with the other side of thefilter 25 connected to the positive potential of the battery 21, andwith the diodes being connected in a direction such as to bereverse-biased when the corresponding one of the MOS FETs 23A, 23B isset in the on (i.e., conducting) state. Capacitors 31A, 31B for noisesuppression are respectively connected in parallel with the diodes 24A,24B. The n filter 25 is formed of a coil 28 and capacitors 26, 27 asshown. These components correspond in function to the correspondinglynumbered components of the first embodiment.

The input signal processing section 34 converts the drive command signalto a drive command voltage, which is supplied from the input signalprocessing section 34 to each of the drive voltage compensation sections113A, 113B. The positive potential (indicated as +B) of the battery 21is supplied to each of the drive voltage compensation sections 113A,113B, while the voltages appearing at the drains of the MOS FET 23A andthe MOS FET 23B are supplied to the drive voltage compensation sections113A, 113B respectively. A pair of compensated drive command voltages,generated as described hereinafter, are produced from the drive voltagecompensation sections 113A, 113B, and are inputted to the phaseprocessing section 135. The phase processing section 135 generates aresultant pair of PWM control signals A and B, having respective dutyratios that are separately determined by the phase processing section135 as described hereinafter, which are applied through the drivecircuits 36A, 36 b to the resistors 37A, 37B respectively, to be eachsubjected to waveform shaping for being converted from a rectangularwaveform to a trapezoidal waveform, as described for the firstembodiment. The resultant waveform-shaped PWM control signals A and Bfrom the resistors 37 a, 37B are applied as respective gate drivesignals to the MOS FETs 23A, 23B respectively, with the advantages of atrapezoidal waveform gate drive signal being thereby obtained asdescribed hereinabove for the first embodiment.

It can thus be understood that this embodiment essentially differs fromthe first embodiment described above in that the motors 122A, 122B ofthe second embodiment have respectively different power ratings, andfurther differs by incorporating the drive voltage compensation sections113A, 113B, and by the functions performed by the drive voltagecompensation sections 113A, 113B in conjunction with the phaseprocessing section 135, as described in the following.

The operation and internal configuration of each of the drive voltagecompensation sections 113A, 113B, and their relationship to the inputsignal processing section 34 and phase processing section 135, will bedescribed referring to the partial system block diagram of FIG. 11, inwhich only those components necessary for describing the operation ofthe drive voltage compensation sections 113A, 113B are shown. In thefollowing, system components which are common to both of the drivevoltage compensation sections 113A, 113B will in general be referred toby the corresponding reference numeral without the A or B suffix, forbrevity of description.

As shown, each drive voltage compensation section 113 is formed of avoltage divider 215, an amplifier 216, and an integrator 217. Thevoltage divider 215 is formed of resistors 218, 219 connected in seriesbetween the positive potential of the battery 21 and ground potential.The amplifier 216 is formed of an operational amplifier 220, with aresistor 221 and capacitor 222 connected in parallel between theinverting input terminal and output terminal, and having the junction ofthe resistors 218, 219 connected to the non-inverting input terminal. Inaddition, the inverting input terminal of the operational amplifier 220is connected via a resistor 223 to the drain of the MOS FET 23. Theintegrator 217 performs both the functions of an integrator circuit anda voltage subtractor, and is made up of an operational amplifier 224having a capacitor 226 connected between the inverting input terminaland output terminal, and having the inverting input terminal connectedvia a resistor 225 to the output terminal of the operational amplifier220. The drive command voltage produced from the input signal processingsection 34 is supplied to the non-inverting input terminal of theoperational amplifier 224.

The phase processing section 135 is formed of a carrier signal outputsection 227 and a phase shifting section 228, and comparators 229A,229B. The functions of these respectively correspond to those of thecarrier signal output section 35A, the phase shifting section 35B andthe comparators 300A, 300B of the phase processing section 35 of thefirst embodiment, described above referring to FIG. 2. The comparators229A, 229B constitute a PWM signal generating section, with thecomparator 229A receiving at its non-inverting input terminal (from theintegrator 224A) a compensated drive command voltage A, which is thedifference between the drive command voltage from the input signalprocessing section 34 and a compensation voltage which is produced fromthe amplifier 216A. Similarly, the comparator 229B receives at itsnon-inverting input terminal (from the integrator 224B) a compensateddrive command voltage B, which is the difference between the drivecommand voltage from the input signal processing section 34 and acompensation voltage that is produced from the amplifier 216B.

The comparators 229A, 229B thereby produce respective PWM controlsignals A and B, whose duty ratios are separately determined by thecompensated drive command voltage A and compensated drive commandvoltage B respectively. The PWM control signals A and B are supplied tothe drive circuits 36A, 36B repectively.

Preferably, in the same way as described hereinabove for the phaseprocessing section 35 of the first embodiment, the amount of phasedifference established between the phase-shifted carrier signal and nonphase-shifted carrier signal by the phase shifting section 228 of thephase processing section 135 is such that each timing of the terminationof a rising edge of one of the PWM control signals A and B (when theseare applied as waveform-shaped gate control signals to the MOS FETs 23A,23B) coincides with the timing of the start of a falling edge of theother one of these PWM control signals.

Each amplifier 216 produces a compensation voltage whose value is equalto the average drive voltage being applied to the corresponding one ofthe MOS FETs 23A, 23B, multiplied by a specific amplification factor, asdescribed in the following. Any change in the difference between thatcompensation voltage and the drive command voltage, appearing at theoutput of the corresponding integrator 224, results in a flow ofcharging current into the capacitor 226 of that integrator.

It is a basic feature of this embodiment that the amplification factorof each amplifier 216 is preset in accordance with the power outputrating of the corresponding one of the motors 122A, 122B. This is donein order to apply respectively different appropriate values of (average)drive voltage to the motors 122A, 122B, such as to substantiallyequalize the respective levels of output power from the motors, andmaintain that equalized relationship irrespective of changes in thedrive command signal from the engine ECU.

Designating the supply voltage value of the battery 21 as B, therespective values of the resistors 218A and 219A as R1, R2, and assumingthat the respective values of the resistors 223A and 221A of theamplifier 216A are also equal to R1 and R2, the value of the referencevoltage B′A is obtained as R2.B/(R1+R2). Designating the average valueof voltage appearing at the drain of the MOS FET 23A as VDA and thevalue of the compensation voltage A that is produced from the amplifier216A as VOA, the following is true:VOA=B′A+(R 2/R 1)(B′A−VDA)=(B−VDA)  (1)

Thus in that case, the output voltage VOA from the amplifier 216A is thedifference between the power supply voltage (applied to one terminal ofthe motor 122A) and the average value of drain voltage of the MOS FETwhich drives that motor (applied to the other term of the motor). Thatis to say, with the above-described relationships of resistor values forthe amplifier 216A, the output voltage VOA is equal to the average valueof drive voltage applied to the motor 122A multiplied by anamplification factor of one.

Hence the duty ratio of the PWM control signal A is determined by acompensated drive command voltage that is obtained by combining theaverage drive voltage of the motor 122A (multiplied by an amplificationfactor that can be arbitrarily preset) with the drive command voltage.With this embodiment, the combining is performed by subtraction ofvoltages, however the invention is not limited to such a method.

Similarly in the case of the drive voltage compensation section 113B,designating the respective values of the resistors 218B and 219B as R3,R4, and assuming that the respective values of the resistors 223B and221B of the amplifier 216B are also equal to R3 and R4 so that theamplification factor of the amplifier 216B is 1, and designating theaverage value of voltage appearing at the drain of the MOS FET 23B asVDB and the output voltage from the amplifier 216B as VOB, the followingequation can be established:VOB=B′B+(R 2/R 1)(B′B−VDB)=(B−VDB)  (2)

In this case the value of the reference voltage B′B is obtained asR4.B/(R3+R4).

Thus for example by adjusting the values of the resistors 218A, 219A,221A, 223A such as to alter the amplification factor of the amplifier216A by a requisite amount, the actual duty ratio of the PWM controlsignal A can be adjusted to a value appropriately different from theduty ratio of the PWM control signal B.

It can thus be understood that, although both of the motors 122A, 122Bare controlled in common by the drive command signal that is suppliedfrom the engine ECU 32, by setting the aforementioned resistance valuesR1 to R4 appropriately, PWM control signals having respectivelydifferent values of duty ratio (determined in accordance with the powerratings of the motors 122A, 122B) can be supplied to the drive circuits36A and 36B, with the MOS FETs 23A, 23B being thereby switched withcorrespondingly different duty ratios. Hence, the amplification factorsof the amplifiers 216A, 216B can be preset such as to apply equivalentlevels of drive voltage to the motors 122A, 122B, i.e., such as toequalize the respective levels of power that are produced by the motors122A, 122 b. That condition of equalized levels of power, to produceequalized levels of air flow from the fans 251, 252, will besubstantially maintained as the duty ratio of the drive command signalsupplied from the engine ECU 32 is varied.

FIG. 12 shows an example of graphs of the relationship between (average)drive voltage applied to a motor and resultant air flow rate from a fandriven by the motor, for the case of a 100 W and a 160 W motorrespectively. As shown, for example the air flow rate produced by thefan of the 160 W motor is 2000 m³/h with a drive voltage ofapproximately 5.8 V, whereas it is necessary to apply a drive voltage of7.0 V to the 100 W motor in order to achieve the same air flow rate.Assuming that the 160 W motor and 100 W motor respectively correspond tothe motors 122A, 122B of the second embodiment, it can be understoodthat the amplification factor of the amplifier 216B can be preset byadjusting the values of the resistors 218B, 219B, 221B, 223Bappropriately. That is to say, the amplification factor would be presetsuch that when a drive voltage of 5.8 V is being supplied to the motor122A, the duty ratio of switching the FET 23B is increased relative tothat of the FET 23A to an extent that a drive voltage of 7.0 V issupplied to the motor 122B. In that way, equivalent levels of (average)drive voltage can be applied to the motors 122A, 122B, such thatidentical values of air flow rate are produced by the cooling fans 251,252.

As a result, greater efficiency of cooling is achieved, signifying thata reduced level of energy is consumed by the apparatus, by comparisonwith the prior art.

Although the second embodiment has been described for the case ofdriving two motors which are controlled in common from a single drivecommand signal, it would be equally possible to drive three or moremotors. In that case, the motors could all have respectively differentvalues of power rating. However for example if two of the motors havethe same rating and one of the motors has a different rating, then theadjustment of duty ratio as described above, to apply equivalent levelsof drive voltage to the respective motors, need only be applied to themotor which has the different rating.

It should also be noted that although the second embodiment has beendescribed for the case of forming the (gate drive) PWM control signalswith a trapezoidal waveform, it would be equally possible to utilize arectangular waveform for these control signals as illustrated in FIG.13, if the level of generated electrical noise is acceptable.

It should also be noted that the invention is not limited to the circuitconfiguration shown in FIG. 11 for establishing respectively differentvalues of duty ratio for driving the motors, and that other forms ofcircuit configuration could be utilized which fall within the scopeclaimed for the present invention, so long as it becomes possible topreset a condition whereby respectively different values of duty ratioare established for driving a plurality of motors having variousdifferent power ratings, whose combined power output level is controlledbased on a single drive command signal, with the respective duty ratiosbeing changed in accordance with changes in the drive command signal insuch a manner as to apply equivalent levels of average drive voltage tothe respective motors, for thereby maintaining respectively identicallevels of output power from the motors, or maintaining respectivelyidentical levels of a parameter such as air flow rate of devices such asfans which are driven by the respective motors.

Moreover, as for the first embodiment, the second embodiment is notlimited to the use of MOS FETs for driving the motors, and could beutilized to control various other types of switching devices such aspower bipolar transistors, IGBTs, etc.

The above description of embodiments should therefore be taken in adescriptive sense and not in a limiting sense.

1. In an inductive load drive apparatus for performing PWM (pulse widthmodulation) switching of current from a DC power source to separatelydrive two inductive loads, a control circuit including two switchingelements which are respectively connected in series between said twoinductive loads and said DC power source, each of said switchingelements controllable for being selectively set in an on state and anoff state for enabling and interrupting a flow of a drive currentthrough the corresponding one of said inductive loads, filter meansconnected in a flow path of a regenerative current which is generatedwhen either or both of said switching elements is set in said off state,and a PWM control signal generating section for generating two PWMcontrol signals for controlling respective ones of said switchingelements, with a duty ratio of said PWM control signals determined basedon an externally supplied drive command signal; wherein said controlcircuit comprises phase control means for controlling the operation ofsaid PWM control signal generating section such that each timing oftermination of a first-direction edge of at least one of said two PWMcontrol signals, whereby a corresponding switching element is changed tosaid off state, coincides with a timing of commencement of asecond-direction edge of the other one of said PWM control signals,whereby a corresponding switching element is changed to said on state.2. The inductive load drive apparatus according to claim 1, wherein saidfilter means comprises a π-configuration filter formed of an inductorand two capacitors.
 3. The inductive load drive apparatus according toclaim 1, wherein said drive circuit means comprises waveform shapingmeans for converting said first and second PWM control signals torespective waveform-shaped PWM control signals having a trapezoidalwaveform, said waveform-shaped PWM control signals having said timingrelationship between said first-direction edges and second-directionedges thereof, and being applied to control corresponding ones of saidswitching elements.
 4. The inductive load drive apparatus according toclaim 1, wherein said switching elements are field effect transistors,and wherein said waveform shaping means comprises a pair of resistorscoupled to supply respective ones of said PWM control signals tocorresponding gate electrodes of said field effect transistors.
 5. Theinductive load drive apparatus according to claim 1 wherein saidinductive loads are constituted by a pair of motors having respectivelydifferent values of power rating, wherein said control circuit comprisesa supply voltage compensation section for applying compensation to saidPWM control signals such that respectively equivalent values of drivevoltage are applied to said motors and substantially identical levels ofdrive power are thereby produced by said motors.
 6. The inductive loaddrive apparatus according to claim 5 wherein said motors are coupled torotate respective cooling fans, and wherein said compensation is appliedsuch that substantially identical values of air flow rate are producedby said cooling fans.
 7. The inductive load drive apparatus according toclaim 5 wherein said control circuit comprises first and second drivevoltage compensation sections for acting on said PWM control signalgenerating section to determine respective duty ratios of said PWMcontrol signals in accordance with said respective values of powerrating and respective values of average drive voltage that are beingapplied to said motors.
 8. The inductive load drive apparatus accordingto claim 7 wherein said drive command signal is expressed as a drivecommand voltage, and wherein each of said drive voltage compensationsections comprises an amplifier circuit for deriving said average drivevoltage as an average difference between a supply voltage of said DCpower source and a voltage applied to the corresponding one of saidmotors from the corresponding one of said switching elements, and formultiplying said average drive voltage by a preset amplification factordetermined based on said power rating of said corresponding motor, toobtain a compensation voltage, and combining means for combining saidcompensation voltage with said drive command voltage to obtain acompensated drive command voltage; wherein said PWM control signalgenerating means is controlled by said compensated drive command voltagefor determining said duty ratio of said PWM control signal whichcontrols said corresponding switching element.
 9. The inductive loaddrive apparatus according to claim 8 wherein said combining meanscomprises means for subtracting said compensation voltage from saiddrive command voltage.
 10. In an inductive load drive apparatus forperforming PWM (pulse width modulation) switching of current from a DCpower source to separately drive a plurality of motors which rotaterespective cooling fans, including at least two motors havingrespectively different values of power rating, a control circuitincluding a plurality of switching elements which are respectivelyconnected in series between said motors and said DC power source, eachof said switching elements controllable for being selectively set in anon state and an off state for enabling and interrupting a flow of adrive current through the corresponding one of said motors, and a PWMcontrol signal generating section for generating a plurality of PWMcontrol signals for controlling respective ones of said switchingelements, with a duty ratio of said PWM control signals determined basedon an externally supplied drive command signal; wherein said controlcircuit comprises a supply voltage compensation section for applyingcompensation to said PWM control signals applied to said switchingelements such that equivalent values of average drive voltage areapplied to said motors, whereby substantially identical values of airflow rate are produced by said cooling fans.
 11. The inductive loaddrive apparatus according to claim 10 wherein said control circuitcomprises a plurality of drive voltage compensation sectionsrespectively corresponding to said motors, responsive to respectivevalues of average drive voltage currently being applied to said motorsfor acting on said PWM control signal generating section to determinerespective duty ratios of said PWM control signals such as to determinesaid equivalent values of average drive voltage.
 12. The inductive loaddrive apparatus according to claim 11 wherein said drive command signalis expressed as a drive command voltage, and wherein each of said drivevoltage compensation sections comprises an amplifier circuit forderiving said average drive voltage as an average difference between asupply voltage of said DC power source and a voltage applied to thecorresponding one of said motors from the corresponding one of saidswitching elements, and for multiplying said average drive voltage by apreset amplification factor determined based on said power rating ofsaid corresponding motor, to obtain a compensation voltage, andcombining means for combining said compensation voltage with said drivecommand voltage to obtain a compensated drive command voltage; whereinsaid PWM control signal generating means is controlled by saidcompensated drive command voltage for determining said duty ratio ofsaid PWM control signal which controls said corresponding switchingelement.
 13. The inductive load drive apparatus according to claim 12wherein said combining means comprises means for subtracting saidcompensation voltage from said drive command voltage.